Two new billboards, standing across from a Nation of Islam mosque and over a hair salon in Roxbury, show Senator Scott Brown shaking hands with President Obama. Underneath the billboards, the state Republican Party has opened an office with workers wearing “Obama supporters for Brown” T-shirts.

In the last week, the senator has met with black pastors in Boston, visited a black church in Dorchester, and called into a black pastor’s ­radio show in Springfield.

It is a highly unusual effort by a Republican to court African-American voters, who are traditionally Democratic. Brown’s campaign is betting that even a small gain could tip the scales in November. But judging from the people who live and work around Brown’s office in the Grove Hall section of ­Roxbury, it is more than a tough sell.

“I don’t think it’s working,” said Richard Green, 42, a bank security guard who works across from Brown’s office. “He’s never been through here, and then, all of a sudden a couple of weeks ago, they all show up.”

Lauda Lopez, 60, who was having a coffee at a nearby Dunkin’ ­Donuts, dismissed Brown’s efforts as an attempt to obscure his differences with the Democratic Party, which she supports.

“Come on, man, that’s a snow job,” she said, with a wave of her hand. “He has not pulled the wool over our eyes. He’s a Republican!”

Brown’s Democratic opponent, Elizabeth Warren, has the backing of most of Boston’s black political and church ­establishment and Obama’s endorse­ment.

‘Just putting up an office is not the same as engaging the community.’

Lists provided by both campaigns show that she has also spent far more time campaigning in communities of color, visiting black churches, and speaking to ethnic media. Last month, for instance, Warren ­attended an NAACP forum at Roxbury Community College that Brown skipped, saying he had already agreed to other ­debates.

“He took the black vote for granted, and Elizabeth Warren has been developing partnerships and going to churches, and the senator is going to be locked out,” said the Rev. Bruce H. Wall, the pastor of Global Ministries Christian Church in Dorchester.

Brown’s campaign said that it is serious about winning black votes and that even a small number could make a difference in a close race. About 7.8 percent of the state’s population is African-American.

“There are votes up for grabs here,” said Benjamin F. Thompson, recently hired to lead the “Obama supporters for Brown” coalition, which has branches in Boston and Springfield. He said the effort begins with letting voters know that, just because they support Obama, does not mean they have to support Warren for Senate.

“We’ve got 15 days, and it’s our hope that when the Latino and African-American community learn about Senator Brown, they will vote for him,” said Thompson, chairman of the Boston Election Commission under Raymond L. Flynn, the former Boston mayor, who has endorsed Brown.

Robert A. Lewis, who runs Brown’s Grove Hall office, said the goal is also to send a message to the Democratic Party that “you can’t take our votes for granted.”

Despite the hurdles Brown faces, some say he needs to at least try to make inroads in the black community.

“There’s no demographic he can leave alone in a presidential year with Obama on the ballot in Massachusetts,” said Representative Michael J. Moran, a Brighton Democrat who pored through voting data as the ­recent chairman of House Redistricting Committee. “He has to go after every demographic. If he leaves any of them alone, he could lose.”

Brown’s efforts have ­impressed some black activists.

“You have to give Senator Brown credit for his willingness to engage a community that is not part of his natural base,” said Kevin C. Peterson, executive director of the New Democracy Coalition, which seeks to increase minority participation in elections. “They may be thinking that to shave a point or two of black votes off the roster for Warren could help him.”

But much of Brown’s campaign in Boston’s black community has taken shape only recent­ly, just weeks before the election. The Obama/Brown billboards — which carry the slogan “Working Together to Get Things Done” — went up a week ago. The Grove Hall office opened two weeks ago.

And the office is not powered by enthusiasm alone. Some workers there, including several who are homeless, are paid $50 a day to hand out leaflets and hold Brown’s signs at intersections.

State Republican officials called the payment a per diem to defray the cost of transportation, meals, and other expenses for the workers.

“Just putting up an office is not the same as engaging the community,” said Larry Ellison, president of the Massachusetts Association of Minority Law Enforcement Officers, who said he voted for Brown in 2010. ­Ellison said he plans to vote for Warren, who has met with his group several times.

“She’s earned my vote by ­being responsive,” he said.

Brown’s campaign argued that he has championed issues of importance to the black community, supporting Metco when he was in the Legislature and helping to ensure the safe return of a Haitian pastor from Boston who was abducted in Egypt last summer. Right after he was elected in 2010, the senator met with black pastors in Boston. Warren, however, was recent­ly endorsed by many of those pastors and opened an ­office in Grove Hall last month.

At Laundry King in Grove Hall, Rose Smith, 57, who was folding her granddaughter’s clothing, said she did not know anyone who plans to vote for Brown.

“No, not in this neighborhood,” she said. “Anybody can open an office anywhere, but to sway voters? I don’t think so.”

Clarification: An earlier version of this story may have left the impression that billboards showing Senator Scott Brown shaking hands with President Obama stand atop a Nation of Islam mosque. They stand across from the mosque.